In recent years, a high occupancy vehicle (HOV) system, which discounts tolls depending on the number of passengers of a vehicle or permits passage of a road only to a vehicle with the passenger number exceeding a predetermined number, has been used. In the HOV system, a technique is used in which the passenger number is counted by photographing a vehicle using an installed camera and performing face detection on the photographed image.
PTLs 1 to 3 are disclosed as a system for counting the number of passengers of a vehicle by face detection. PTL 1 discloses a technique of counting the number of passengers of a vehicle by detecting a profile of a person. PTL 2 discloses a technique of measuring the passenger number by detecting persons and estimating at which positions in a vehicle the persons are on board. PTL 3 discloses a technique of counting the passenger number using a movement amount of a vehicle and a person detection result.